Process for processing electrophotosensitive layers



May 28, 1968 PROCESS FOR PROCESSING ELECTROPHOTOSENSITIVE LAYERS LOG. V (VOLT) RECOVERING RATIO FOR INITIAL SURFACE POTENTIAL SATORU HONJIO I 3,385,699

Filed Juiy 20, 1964 AFTER PLACED ISO- IN DARKNESS DIRECTLY AFTER PRE- EXPOSED 5 26 4b 6b 86 IOIO v CORONA DISCHAIRGING TIME (sec) FIG.

\NON-CURED LAYER FIG. 2

IN VENTOR.

Y 5 1 Hohji o United States Patent 3,385,699 PRUCESS FOR PROCESSING ELECTRO- PHQTOSENSITIVE LAYERS Satoru Honiio, Orlawara-shi, Japan, assignor to Fuji Shashin Film Kahushiki Kaisha, Kanagawa-ken, Japan, a corporation of Japan Filed July 20, 1964, Ser. No. 383,645 Claims priority, application Japan, July 25, 1963, $869,489 6 Claims. (Cl. 96-1) ABSTRACT OF THE DISCLOSURE An electrophotosensitive layer containing zinc oxide and an uncured thermosetting resin is treated to form a Visible image on said layer. Thereafter, the thermosetting resin is cured whereby the developed image is permanently fixed.

This invention relates in general to a process for making visible electrostatic images and in particular to a process for processing electrophotosensitive layers for making visible electrostatic images.

It is well known that when an electrophotosensitive layer containing zinc oxide as the photoconductive material is subjected to a light exposure, the electric resistance of the electrophotosensitive layer is lowered and the layer is, after ceasing the light exposure, maintained at a lowered state of electric resistance for a long period of time. In electrophotography wherein such an electrophotoconductive layer is charged and an electrostatic latent image formed on the charged layer is converted to the corresponding visible image, if the electrophotosensitive layer is light-exposed prior to charging the surface potential of the layer directly after charging is lowered and the attenuation rate of the potential of the layer in a dark place is increased, which is an undesirable phenornenon for electrophotography and is usually called light fatigue or pre-exposure effect.

The light fatigue (hereinafter is called simply fatigue) or the pre-exposure effect on the electrophotosensitive layer can be removed by placing the layer in a dark place for a long period of time, heating the layer in a dark place, or exposing the layer to corona discharge (e.g., to negative corona discharge in the case of using a zinc-oxide electrophotosensitive layer).

Since corona discharge has a very strong action for removing the fatigue of an electrophotosensitive layer, the degree of fatigue of an electrophotosensitive layer can be determined more exactly by observing the form of a charging time-surface potential curve than by measuring the charged potential on the layer. That is, an electrophotosensitive layer placed in a dark place for a long period of life shows a very steep rising slope in the charging time-surface potential curve whereas a light-exposed or fatigued electrophotosensitive layer shows an extremely gentle slope in the curve.

By using such an observation of a charging time-surface potential curve for determining the degree of fatigue and the recovering rate from fatigue, the inventor has investigated various Zinc-oxide electrophotosensitive materials and has found that the recovering rates of electrophotosensitive materials from fatigue are markedly influenced with the kinds and states of binders incorporated in zinc oxide in the electrophotosensitive materials, which are shown together with an example of the above-mentioned charging time-surface potential curve in the accompanying drawings in which:

FIG. 1 is a graph showing the influence of pre-exposure on a rising curve by charging of an electrophotosensitive ice layfr using zinc oxide as the photoconductive material; an

FIG. 2 is a graph showing recovering characteristics from fatigue of a cured electrophotosensitive layer and a non-cured electrophotosensitive layer.

As shown in FIG. 1, an electrophotosensitive layer having a delayed recovering rate from fatigue, viz., a pre exposed electrophotosensitive layer shows a gentle rising slope in the charging time-surface potential curve and no matter how long the charging procedure (corona discharging) is continued, the surface potential of the layer only approaches a saturated potential value, that is, does not exceed the saturated value, which is equal to or lower than about a half of a saturated potential value of the layer placed in a dark place.

An electrophotosensitive layer having a high recovering rate from fatique shows also a gentle rising slope in the charging time-surface potential curve when it is preexposed, but the gentleness of the slope is weak as compared with the above case and the saturated potential value is nearly that of the layer placed in a dark place.

According to the inventors investigation, it has been found that among various binders a silicone resin gives the highest recovering rate from fatigue and other various resins, such as, an alkyd resin, an epoxide resin, a vinyl chloride-vinyl acetate copolymer, a styrenebutadiene copolymer, etc., are inferior to the silicone resin in recovering rate from fatigue independently of the characteristics that they will show when placed in a dark plac However, it has been also found that in the case of using a thermosetting resin as the binder, the recovering rat from fatigue of an electrophotosensitive layer containing the thermosetting resin in an non-cured state is higher than that of the layer containing the resin in the cured state.

It has been further found that the recovering characteristics of an electrophotosensitive layer almost same as that of the case where a silicone resin is used as a binder can he obtained by using a non-cured themrosetting resin as a binder as well as by selecting a suitable compounding ratio and mixing procedure with zinc oxide. As one of these methods, a silicone resin compatible with a thermosetting resin may be suitably mixed.

Now, -a thermosetting resin in a non-cured state has in general a very low polymerization degree and, therefore, the non-cured thermosetting resin causes many troubles when it is adopted as a binder for an electrophotosensitive layer through the use of such a resin is preferable in a point of recovering from fatigue.

For example, there are the following troubles. That is, when a non-cured thermosetting resin is used as a binder for an electrophotosensitive layer of office copying papers,

(1) As the coated film or layer is soft and retains an adhesive property, the copying paper is liable to adhere to other one or they cause so called locking when they are stored for a long period of time or in a comparatively high-temperature circumstance, and

(2) By the same reasons the surface of the copying paper is liable to be mechanically injured and stained; when the resin is used as a binder for an electrophotosensitive layer as a master for olfset printing,

(3) A wetting liquid penetrates easily into the coated film or the photosensitive layer, which results in reducing the life or durability of the master during printing; and when an electrophotosensitive material containing such a non-cured thermosetting resin as the binder is applied for electrophotosensitive marking for working conductive materials such as metals,

(4) As the coated film or layer applied on the conductive material is soft, the film or layer is easily in- 3 jured mechanically, from where the conductive material is apt to rust and (5) In the case where thus electrophotographically marked conductive material is cut by, e.g., a gas flame, the layer containing the non-cured resin is softened adjacent to the gas heated portion by the heat of the gas flame, which makes the operation diflicult as well as spoils the working object.

Therefore, an object of this invention is to provide a process for producing an electrophotosensitive layer having a lowered pre-exposure effect or a high recovering power from fatigue.

Another object of this invention is to provide a process for producing an electrophotosensitive layer having a lowered pre-exposure effect and improved mechanical strength by using a non-cured thermosetting resin as the binder.

Still another object of this invention is to provide a process for processing an electrophotosensitive layer containing a non-cured thermosetting resin as the binder to give the electrophotosensitive layer having a visible image and having an improved mechanical strength.

A further object of this invention is to provide improved electrophotosensitive materials suitable for various electrophotographic purposes, such as, ofiice copying papers, offset-printing masters, electrophotographic marking on conductive materials, and the like.

These and other objects of this invention will become apparent from the following descriptions and claims.

The inventor has found that a better result can be obtained in the case of using a non-cured setting or thermosetting resin as a binder for an electrophotosensitive layer by maintaining the resin in its non-cured state in the photosensitive layer during an electrophotographic treatment and adding, after an image is fomed on the layer, a curing accelerator, a curing catalyst, etc., to the layer (it is of course effective to heat the layer besides). The addition of such a curing agent may preferably be done in practice by dissolving or dispersing it in a solvent for fixing an image, but the treating agent may be added to the layer in an addditional step. For example, in the case of using as a binder an alkyd resin which can be cured as is well known by the action of an organic peroxide and a salt of a heavy metal, such as, manganese and cobalt even at room temperature, one of them may be contained preliminary in an electrophotosensitive layer and after the end of the electrophotographic treatment another may be added to the layer by spraying or impregnating, or both treating agents may be added after.

Even though it is true that the recovering characteristic from fatigue of a zinc-oxide electrophotosensitive layer containing a thermosetting resin other than silicone resins are mostly inferior, even the thermosetting resin is contained in a non-cured state, to those of the layer containing a silicone resin as a binder, the invention has an advantage that the restriction of an electrophotosensitive layer to be conducted according to the process of this invention to light-exposure conditions prior to charging or pre-exposure conditions can be loosened as compared with the case where the electrophotosensitive layer contains as a binder the cured thermosetting resin.

By using the process of the present invention, all the troubles mentioned before can be improved or removed at every point and in particular the invention shows a very remarkable effect in increasing the printing power in offset printing when the invention is applied for processing the offset printing master, and there are no troubles for offset printing in the case of curing the layer or the resin as the post-treatment.

In the case of electrophotographic marking, further, the rust preventing power and weathering resistance of a conductive material applied with an electrophotosensitive layer having a desired image on it can be improved by the process of this invention. Furthermore, it frequently occurs in the case of cutting, after the electrophotographic marking on a conductor such as a steel plate, the steel plate by means of an automatic gas cutting machine placed on the steel plate that if the thermosetting resin is not cured the non-cured layer is softened by the heat of the gas flame and the wheels of the gas cutting machine slip on and injure the softened portion of the layer, which results in causing rust formation. However, it has been found by a practical cutting operation that those drawbacks are improved or completely removed by curing the layer in accordance with the process of this invention.

In order to increase the hardness of an electrophotosensitive layer after the end of the electrophotographic process in accordance with the process of this invention, instead of adding only an curing agent or catalyst to the layer, a hard film-forming material or a material which can be copolymerized or condensated with the binder in the layer may of course be added besides.

The invention is further illustrated by the following examples without being restricted thereto.

Example 1 Component: Parts 'by wt. Photoconductive zinc oxide powder Short-oil type alkyd resin varnish (soybean oilmodified; oil length 40%) (non-volatile matter 50%) 30 Low molecular weight silicone resin varnish (non-volatile matter 60%) 30 Toluene 20 Methyl ethyl ketone peroxide 0.5

The above composition was mixed for 20 hours in a porcelain ball mill. Thus prepared homogeneous liquid was diluted with toluene and applied by spraying on a steel plate pre-coated with a wash primer in a coating thickness (after dried) of 2011.. The plate was allowed to evaporate oh the solvent for 20 hours at room temperature. The layer was placed in darkness for 1 hour and exposed to a corona discharge, and the surface potential of the charged layer was determined to be about 400-450 volts by measuring. The layer was subjected to an image exposure, a cascade development, and then fixing according to a conventional electrophotographic process. As the fixing liquid, the following mixture was used.

Ingredient: Parts by wt. Cobalt naphthenate 2 Acetone 30 Trichloroethylene 40 Toluene 30 The liquid was uniformly applied by spraying on the surface of the developed electrophotosensitive layer in an amount of 50-60 cc./m.

The developer and the electrophotosensitive layer were swelled or dissolved by the action of the fixing liquid whereby the image was fixed and simultaneously cobalt naphthenate was penetrated into the layer. When thus treated layer was placed for about 10 hours at room temperature, it became solvent-insoluble. This resulted from the alkyd resin and the silicon resin in the layer being cured by the co-operation of the methyl ethyl ketone peroxide contained in the layer and the cobalt naphthenate in the fixing liquid.

For comparison, an electrophotosensitive layer was prepared by applying on a steel plate the above-mentioned coating liquid but added with 1 part by weight of cobalt naphthenate and after 24 hours the characteristics were measured, which was compared with the results obtained by measuring the characteristics of the layer prepared as mentioned above in the example prior to fixing, that is, the characteristics of the layer in the state before spraying the fixing liquid containing cobalt naphthenate. These results were as follows:

Non-cured Cured The relative sensitivity is a reciprocal of a light-exposure quantity required for decreasing the surface potential to a half of the surface potentialprior to the light exposure.

As shown in the above table, it is clear that the characteristics of the cured layer when it was placed in a dark place for a long period of time is superior in every point to those of the non-cured layer but the difference between them about the recovering characteristics from fatigue is decisive, that is, the latter layer is far superior to the former in this point. This shown more in detail in FIG. 2.

FIG. 2 shows clearly the difference of the recovering power between non-cured and cured layers. In FIG. 2, two curves corresponding to non-cured and cured layers are shown about the recovering ratios of the initial surface potential to a period placed in a dark place after pre-exposed to 5,000 lux fluorescent lamp for 5 minutes, that is, the ordinate of FIG. 2 represents a relative value of the surface potential of the layers subjected to the preexposure, the surface potential (shown in the above table) after placed in a dark place being defined to be 100. From FIG. 2, it is concluded that it takes several hours to 10 hours for the cured layer to recover completely from fatigue, while the non-cured layer is recovered from fatigue practically in several minutes. It might of course be considered that such an effect might be caused by the addition of methyl ethyl ketone peroxide and cobalt naphthenate, but this consideration is denied considering the fact that the above effect can also be obtained by curing the layer by heating without adding cobalt naphthenate and that in the case where the layer is added with cobalt naphthenate but is not added with methyl ethyl ketone peroxide, whereby the rate of the curing rate is lowered, the recovering rate from fatigue is high in the layer directly after coating.

Similar effect was always observed about other Zincoxide electrophotosensitive layers containing other thermosetting resins as the binders. Also, it was ascertained that there is the same tendency in a zinc-oxide electrophotosensitive layer containing the thermosetting resin together with a plasticizer or a thermoplastic resin if the hardness of the layer is dominantly influenced by curing of the thermosetting resin.

From the above explanation, the advantage of maintaining the layer in a non-cured state until the image forming process is finished will be understood, and in the following example, the advantage will be explained of curing the layer after the image forming process is finished.

EXAMPLE 2 A shot-blast steel plate coated with the electrophotosensitive layer as in Example 1, which was cured after fixing and a shot-blast steel plate coated with the same layer in Example 1, which was maintained in the noncured state without the addition of the catalyst, were placed in an aqueous 3% NaCl solution for 100 hours. The swelled area and the rusted area of the non-cured layer were about /3 of those of the cured layer.

In the case of conducting gas cutting, the portions closely adjacent to the cut lines were heated, which resulted in softening the portion of the non-cured layer, and the softened portion of the non-cured layer was injured when the portion was contacted with a torchsupporting means or with an operator, which caused a rust formation when the plate was exposed in air for a long period of time. On the other hand, the cured layer was softened a little by heating and there were almost no such drawbacks as in the case of the noncured layer.

EXAMPLE 3 Component: Parts by wt. Photoconductive zinc-oxide powder Epoxy ester-modified alkyd resin varnish (nonvolatile matter 50%) Toluene 20 Methyl ethyl ketone peroxide 0.5

The coating liquid having the above composition was applied on a steel plate as in Example 1 to a dried thickness of 3035,u. After drying, the plate was placed for 20 hours in a dark place. While, the fixing solution same as in Example 1 was sprayed onto the above photosensitive layer and after dried at room temperature the plate was also placed for 20 hours in a dark place. It was observed in this case that workability for gas cutting of the layer was much improved as in Example 2 by the increase in the hardness of the layer.

The results of the tests for comparing the characteristics of the non-cured layer and the cured layer were as follows:

N on-cured Cured EXAMPLE 4 The composition as in Example 3 was applied on two art papers and after forming images on the layers electrophotographically one was fixed with a fixing liquid containing a catalyst, cobalt naphthenate and another was fixed with a fixing liquid without containing cobalt naphthenate.

Thus obtained images were treated with a hydrophilic converting liquid mainly consisting of potassium ferrocyanide and ammonium chloride and lithoprinting was conducted by using thus treated papers respectively.

While a wetting liquid began to penetrate into the paper having non-cured layer when about 300 sheets of prints was printed, whereby the paper was injured and the portion was stained with a printing ink, no impregnation of the wetting liquid into the paper having the cured layer occured even after above 700 sheets of prints was printed.

What is claimed is:

1. A process for processing an electrophotosensitive layer provided on a conductive substrate and containing photoconductive zinc oxide powders and a thermosetting resin selected from the group consisting of alkyd resin and epoxy ester resin, which comprises maintaining said thermosetting resin in said layer in a non-cured state until the formation of a visible image on said layer by an electrophotognaphic process is finished, subjecting said layer to uniform charging, image exposure and development with dry developing powders, and after finishing the image formation, uniformly applying on the layer a solution of a curing agent selected from the group consisting of a curing catalyst, a curing accelerator and a material which can effect polymerization reaction or condensation reaction with the resin in the layer dissolved in a volatile organic solvent which can dissolve both the curing agent and the dry developing powders employed, whereby the thermosetting resin is cured and the developed image is permanently fixed.

2. The process according to claim 1, wherein the curing operation is carried out while heating the layer.

3. The process according to claim 1, wherein said curing agent is added after fixing in an additional step.

4. The process according to claim 1, wherein the ad- 7 8 dition of said curing agent is carried out at fixing by References Cited adding the agent t0 a fixing agent. UNITED STATES PATENTS 5. The process according to claim 1, wherein said curing agent consists of a two-agent system, one being 3192043 6/1965 Metcalte et a1 96-1 contained in said electrophotosensitive layer and another 5 3,226,227 12/1965 W911i being added to a fixing agent 3,291,738 12/1966 SCIELmbI 25242.1

6. The process according to claim 5, wherein one of said curing agents is methyl ethyl ketone peroxide and NORMAN TORCHIN Pnma'y Exammer' the other is cobalt naphthenate. C. E. VAN HORN, Assistant Examiner. 

